The present invention relates to a video camera capable of controlling the white balance of video signals provided by an image pickup device, such as a CCD (charge-coupled device), and a white balance control method.
A video signal processing circuit included in a related art video camera integrates a chroma signal provided by a chroma signal processing unit which processes video signals provided by an image pickup device to detect the color temperature of an object for white balance control. A signal processor, such as a microcomputer, processes the integral of the chroma signal to calculate the color temperature of the object.
FIG. 3 is a block diagram of the video signal processing circuit of such a related art video camera. The video signal processing circuit has a CCD as a image pickup device, a luminance signal processing unit 10 for producing a luminance signal and a chroma signal from the signals of the CCD output provided on a plurality of lines, and a chroma signal processing unit (CR) 20.
In the video signal processing circuit, a first-line (reference) signal of the CCD output (CCD Sig), and a second-line signal by delaying a (1H preceding) signal of the CCD output by one horizontal scanning interval (1H) through a delay means (1HDL) 1 which delays the signal of the CCD output by one horizontal scanning interval (1H) are given simultaneously to a luminance signal processing unit (Y) 10.
The luminance signal processing unit 10 has a vertical aperture circuit (Vap) 11 for detecting the difference between the first-line signal and the second-line signal. The vertical aperture circuit 11 produces and provides a vertical edge signal (V edge)of an input video signal. The vertical edge signal and the first-line signal are given to a luminance signal processing circuit included in the luminance signal processing unit 10, and then the vertical edge signal and the first-line signal are processed by the luminance signal processing circuit.
The first-line signal and the second-line signal are given to a primary color generation circuit 21 included in the chroma signal processing unit 20. The primary color generation circuit 21 produces a first color difference signal (Cr) and a second color difference signal (Cb) from the first-line signal and the second-line signal, respectively. Therefore, the primary color generation circuit 21 has a first interpixel differential circuit 21-1 that receives the first-line signal, and a second interpixel differential circuit 21-2 that receives the second-line signal. The first color difference signal Cr and the second color difference signal Cb are given to the primary color generation circuit 21 that generates R, G and B primary color signals. The R, G and B primary color signals are given to a built-in chroma signal processing circuit.
The chroma signal processing unit 20 gives the R, G and B primary color signals generated by the primary color generation circuit 21 to a primary color integration circuit 3 (CR-INT), and then the primary color integration circuit 3 integrates the R, G and B primary color signals. Integrals obtained by integrating the R, G and B primary color signals by the primary color integration circuit 3 are given to the microcomputer, and then the microcomputer processes the integrals for color temperature detection to calculate the color temperature of the object. The white balance is adjusted according to the calculated color temperature to display an image in a natural color tone.
An operation of the primary color generation circuit 21 for generating the first color difference signal Cr and the second color difference signal Cb will be explained with reference to FIG. 4.
Shown in FIG. 4 is a portion of a CCD 30 having a color filter array of a checkered complementary color system. An Mg (magenta)filter, a Ye (yellow) filter, a G (green) filter and a Cy (cyan) filter are arranged on the first line. A G filter, a Ye filter, an Mg filter and a Cy filter are arranged on the second line.
Since Mg, Ye and Cy are expressed by R+B, R+G and B+G, respectively. Therefore, a first-line first pixel signal 1S1 is Mg+Ye=2R+G+B, a first-line second pixel signal 1S2 is G+Cy=2G+B, a second-line first pixel signal 2S1 is G+Ye=2G+R, and a second-line second pixel signal 2S2 is Mg+Cy=2B+G+R. A signal of the CCD output, i.e., a reference signal, is provided on the first line, and a (1H preceding) signal of the CCD output is provided on the second line.
The first-line first pixel signal 1S1 and the first-line second pixel signal 1S2 are given to the first interpixel differential circuit 21-1 of the primary color generation circuit 21, and the difference between the pixel signals lS1 and 1S2 is calculated and the first interpixel differential circuit 21-1 provides: EQU 1S1-1S2=(2R+G+B)-(2G+B)=2R-G
The signal 2R-G is the first color difference signal Cr.
The second-line first pixel signal 2S1 and the second-line second pixel signal 2S2 are given to the second interpixel differential circuit 21-2 of the primary color generation circuit 21, and the difference between the pixel signals 2S1 and 2S2 is calculated and the second interpixel differential circuit 21-2 provides: EQU 2S2-2S1=(2B+G+R)-(2G+R)=2B-G
The signal 2B-G is the second color difference signal Cb.
The video signal processing circuit of the related art video camera, however, produces a luminance step between the first line and the second line if there is a vertical edge. Therefore, spurious R, G and B primary color signals are produced from the first color difference signals Cr generated from the signal provided on the first line, and the second color difference signal Cb generated from the signal provided on the second line.
Causes of the spurious color signals will be explained with reference to FIG. 5. Suppose that the signal level of the signal of the CCD output rises to the left as indicated by the arrow in a left part of FIG. 5. Then, a vertical edge level step develops between the first line and the second line, if there is a vertical edge.
Consequently, the level of the first color difference signal Cr generated by calculating the difference between the first-line first pixel signal 1S1 and the first-line second pixel signal 1S2 by the first interpixel differential circuit 21-1 of the primary color generating circuit 21 is low because level of the signal of the CCD output provided on the first line is low, while the level of the second color difference signal Cb generated by calculating the difference between the second-line first pixel signal 2S1 and the second-line second pixel signal 2S2 by the second interpixel differential circuit 21-2 of the primary color generating circuit 21 is high because level of the signal of the CCD output provided on the second line is high.
Since the respective levels of the first color difference signal Cr and the second color difference signal Cb are different from each other, the R, G and B primary color signals generated by processing the first color difference signal Cr and the second color difference signal Cb are spurious color signals different from R, G and B primary color signals representing the original color of the object.